Abrasion resistant coating for aluminum base alloy and method

ABSTRACT

The method for applying an abrasion resistant coating on the surface of an aluminum or aluminum alloy conductor. After cleaning the conductor surface to remove dirt, grease, and oxides and to make the surface uniformly active, a plurality of coatings of metals are applied including an inner coating of tin or zinc, intermediate coatings of bronze or copper, and nickel, and an outer coating of tin or silver.

United States Patent Ricks 5] Oct. 28, 1975 ABRASION RESISTANT COATING FOR 2,654,701 10/1953 Calderon et al. 204/38 A x ALUMINUM S ALLOY AND METHOD 2,995,814 8/1961 Chamness 204/33 X 3,108,006 10/1963 Kenedi et al. 204/33 X Inventor: Herbert Ricks, t g 3,455,014 7/1969 Beyer 204/40 x [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Primary Examiner-L. Dewayne Rutledge 22 Filed: p Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmL. P. Johns [21] Appl. No.: 399,766

Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. B 399,766.

US. Cl 29/197; 117/71 M; 204/33 Int. Cl. C23B 5/00; C23B 5/22 Field of Search... 117/71 M; 204/33, 40, 38 B;

References Cited UNITED STATES PATENTS l/l953 Heiman 204/38 A X [5 7 ABSTRACT 3 Claims, No Drawings ABRASION RESISTANT COATING FOR ALUMINUM BASE ALLOY AND METHOD CROSS REFERENCE TO RELATED APPLICATION This application is related to the invention disclosed in the application of Julius Toth and Edward M. Walker, Ser. No. 364,173, filed May 25, 1973.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an abrasion resistant coating for an aluminum and aluminum alloy conductor and to a method for applying the coating.

2. Description of the Prior Art Since 1945, the substitution of aluminum for copper as electrical conductor material has continually increased until at the present time over 50% of the conductors are made of aluminum. This trend is due to the almost unlimited availability of aluminum and its much lower cost compared to copper. The substitution of aluminum for copper in bus bars, however, can result in a cost saving only if a low cost process for plating the contact joints is available. The ever present highresistivity oxide coating on the aluminum must be replaced with a high conductivity coating such as silver plate if the aluminum conductors are to be treated like copper conductors with respect to bolting and unbolting during assembly. However, for stab type contacts to bus bars in bus duct and load center apparatus, it is necessary to have the aluminum coated with a good wear resistant coating such as copper or nickel plus a coating such as silver, tin, or cadmium for lubrication and low contact resistance.

The application of protective coatings for various purposes on the surface of members composed of aluminum and aluminum alloys is well-known in the art such as disclosed in US. Pat. Nos. 2,377,606; 2,947,639; and 3,274,021. Generally, those patents disclosed a method for providing a tightly adherent tin coating on an aluminum surface.

Where specific environmental problems are incurred, additional protection is necessary. In the past a coating consisting of layers of zinc, copper, and silver plating on aluminum bus bars has left much to be desired for environmental protection; improved corrosion resistance in specialized areas has been indicated.

Associated with the foregoing has been a problem of fretting due to either mechanical wear or corrosion. In one form fretting is wear resulting from oscillating or vibratory motion of a limited amplitude. Another form of fretting results from actual wear, frictional changes,'

variations in electrical resistances, electrical noise, and fatigue failure. Aluminum is particularly susceptible to fretting corrosion which is the formation of hard abrasive oxides (A1 as debris and by an excessive rate of oxygen absorption. As a result, there has been a problem of providing aluminum members, such as bus bars or conductors, with a protective coating of a suitable type of which abrasion and corrosion are avoided.

SUMMARY OF THE INVENTION Generally, it has been found in accordance with this invention that the foregoing problems may be overcome by providing an aluminum conductor or bus bar with a multiple layered coating of various metals applied in a given sequence, which sequence consists essentially of an inner layer of tin or Zinc, intermediate layers of bronze or copper, and nickel, and an outer layer of either tin or silver. Where the conductor or bus bar is subject to vibrations, an outer coating of silver instead of tin is preferred.

The invention also comprises a method for applying the several layers in the indicated sequence.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the present invention in practice the new method is carried out in the following sequential manner:

1. Preliminarily cleaning the surface of an aluminum member to be coated with a caustic solution to remove oxides, dirt, grease, and the like;

2. Rinsing the surface in water;

3. Cleaning the surface with an acid;

4. Rinsing the member in water;

5. Applying an inner coating of tin or zinc to the surface;

6. Applying a copper-base alloy to the tin coating or copper strike on the zinc coating;

7. Rinsing the member in water;

8. Applying a coating of nickel on the copper-base alloy or copper coating;

9. Rinsing the member in water;

10. Applying an outer coating of tin or silver on the coating of nickel, and

11. Rinsing the member in water and air drying The method is applicable to pure aluminum and a wide range of aluminum alloys such as aluminum alloy numbers 6061, 6101, 2024, 5052, 7075, 356, 360, and

The first step comprises cleaning the surface with a caustic solution to remove oxides, dirt and the like, and removes about /2 to 1 mil of aluminum surface. The preferred solution forthis purpose is about 8 to 10 ounces of sodium hydroxide per gallon of water at F which is applied to the aluminum surface for a time of from about A to 1 minute. Other alkali metal hydroxides such as potassium hydroxide may be used. A chelating agent is preferably added to the solution to slow down the action: :A common chelating agent is ethylenediaminetetraacetic acid (EDTA). The advantage of the chelating agent is to maintain the material removed from the aluminum surface in solution.

The second step is a water rinse to remove all traces of the caustic solution of the first step because the third step involves an acid solution. After the member is cleaned with a caustic solution, a thick viscous coating or smut of sodium zincate (Na ZnO adheres to the member. This coating is removed better by a double rinse in water.

The third step is a surface cleaning or desmutting operation with an acid solution such as a 1:1 mixture of nitric acid (67%) in water for the purpose of deoxidizing and removing impurities in aluminum such as iron and manganese. Although it is not necessary, about 1 to 5% hydrofluoric acid is preferably added to make the aluminum surface more uniformly active and to prevent blistering of some subsequently deposited metal, such as tin. The blistering often occurs due to poor metal preparation.

The fourth step is another rinse in water to remove all traces of acid of the previous rinse, because the next step involves an alkaline bath.

The fifth step has the dual purpose of activating the aluminum surface and depositing a thin layer of tin or zinc. In this step, where the thin layer is tin, the aluminum member is immersed in aqueous solution, the primary constituent of which is at least one tincontaining compound selected from the group consisting of a stannate of an alkali metal, such as sodium stannate and potassium stannate, together with a chelating agent such as EDTA. The composition of the solution is shown in The constituents, sodium stannate and potassium stannate, may be used either alone or mixed to make up the required composition. Sodium stannate and potassium stannate used here and in subsequent steps may be used either in the anhydrous or the trihydrate form; it being immaterial, because the operating conditions required the solution to be maintained at an elevated temperature of from 75 to 85F with a preferred temperature of 80F. The immersion time may range from 15 to 45 seconds with a preferred time of 30 seconds. A partial or wholly substitute chelating agent is preferably used for stabilizing the time to prevent it from settling to the bottom of the container. It is a mixture of zinc acetate, Zn (C H O in an amount of about 2 ounces per gallon of water and m-cresolsulfonic acid in the amount of about 4 ounces per gallon of water. Thus, by step 5 the surface of the aluminum member is completely activated and the protective layer of tin is deposited. The weight of tin deposited varies from about 0.1 to 0.3 mg/in On the other hand in step 5 where the thin layer is zinc instead of tin, the aluminum member is immersed in an aqueous solution having at least one zinccontaining compound, such as zinc oxide and sodium hydroxide. The immersion time may range from to 45 seconds with a preferred time of about 30 seconds at a temperature of from 75 to 85F with a preferred temperature of 80F. The composition of the zinc solution is shown in Table II as follows:

Table II Activating & Zinc Plating Bath (Ounces per gallon of water) Chelating Agent The chelating agent is preferably used as a stabilizer of the solution.

The sixth step is the application of a coating of copper or a copper-base alloy. Where in step 5 tin is applied, a coating of a copper-base alloy such as a bronze strike is applied on the tin in step 6. However, where zinc is applied in Step 5, a copper strike is applied in step 6. Where tin is applied, a rinse between steps 5 and 6 is unnecessary because the solutions of both steps are alkaline. But where zinc is applied, a rinse is preferred before the application of copper. The sixth step is ac complished by immersing the aluminum member into an electrolyte consisting essentially of a stannate of an alkali metal such as sodium stannate and copper cyanide. A partial or complete substitute for sodium stannate may be potassium stannate. The constituents and the amounts used for the bronze strike are shown in Table III as follows:

Table III Bronze Stn'ke (Ounces per gallon of water) Preferred Range Constituent Composition Composition Na SnO 7-1 1 and/or K SnO 5 4.5-5.5 CuCN 4 3-5 KCN (Free) 9 7-11 KOH (Free) 1.5 1-1.75

Table IV Operating Conditions Condition Preferred Range Time 30 seconds 15-45 seconds Temperature 140F -1 45F Cathode Current density 30 ASF 20-30 ASF Anode Current density 15 ASF 10-20 ASF pH 11.75 11.5-12.5 Anodes copper The thickness of the bronze coating may vary from about 0.00001 to 0.00003 inch.

Where the sixth step is an application of a coating of copper instead of bronze, the constituents and other parameters of the copper solution are listed in Table V as follows:

Table V Copper Strike Conditions Preferred Range Constituent Composition Composition CuCN 5.6 oz/gal 4-6 oz/gal NaCn 6.4 odgal 5-7 ozlgal Rochelle Salt 8 oz/gal 6-10 0d 1 Temperature 1 20- 1 30"] l 1 5-1 35 Table V-Continued I y -Cntinued Watts Type Bath Copper Strike Conditions High-pH Preferred Range Current density I 25-l00 ASF Constituent Composition Composition An'chlonde Bath Current Density 25 ASP 20-30 ASF ggilfgf fi 2 2 2 5 PH 98-107 PH 45 Anodes Copper Temperature 130-l60F Current density 25-100 ASF 1O Fluoborate Bath If the temperature is too high (above 135F), the zinc g f flgoborate Z a layer is dissolved off. If" the pH is too high-above 10.5, a am 4.? g the zinc layer is dissolved by the alkali. E p ure 20158022 urrent enslty The seventh step is a water rinse for the purpose of The ninth stepvis a water rinse removing the bronze or copper str1ke solutions and for preventing contamination of the nickel plate solution which is the eighth step. The tenth step is the application of tin or silver as The eighth step is the application of a relatively thick preferred. A rinse in two baths of cold water is required layer of nickel on the bronze or copper strike. The alubefore plating of tin or silver. minum member is immersed in electrolyte having the Where the choice is tin, the copper plated aluminum constituents listed in Table VI as follows: member is immersed into the plating solution having Table VI the formula and operating conditions set forth in Tables VIII and IX as follows:

Nickel Plate Table VIII (Ounces per gallon of water) Constituent Preferred Range Tm Plate Nickel sulfamate 50-60 20-70 (Ounces P gallon 0f Water) Boric Acid 4-4.5 3.5-5 f rre Ra g A i pini Agent Q06 0 05 Q 07 Constituent Compositlon Composition KOI'I 2 l-4 K SnO and/or The thickness of the resulting nickel plate ranges Na sno 28 26-28 from about 0.0001 to 0.0005 inch, the preferred thick- Tm metal equwalem Table ness being 0.0002 inch. The operating conditions for Operating Conditions the electrolyte bath are listed in Table VII as follows: Range P f d Temperature "F 130-180 140F Voltage 3-6 Table VII current density 30-160 ASF 75 Annode to Cathode ratio Never more than 1:2 Opemmg Commons 40 Time to deposit .0001 to .0006 in. of plating at Preferred R ng recommended current density, minutes 2-4 2 pH 5-5.5 3-5.5 Temperature F 120 1 10-130 Current 30 ASP 30-40 The thickness of the outer tin plate or coating may range from 0.0001 to 0.0006 inch, the preferred thick- Although nickel plated from the nickel sulfamate is r1885 being (3-0005 inchdi l d, i k l m b l t d f the l ti Where the outer plating or coating is silver instead of such as: tin, the copper-plated aluminum member is placed in 5 the silver plating electrolyte having a formula and control conditions as set forth in Tables X and XI as fol- Watts Type Bath lows: High-pH Table X Nickel sulfate 32 oz/gal Nickel chloride 6 oz/gal Boric acid 4 oz/gal Silver Plate F (Ounces per gallon of water) Temperature 1 Constituent Preferred Range Current denisty 20 -l00 ASF Hard Nickel Bath AgCN Q7 @5418 Nickel sulfate 24 oz/gal 60 Na CN 9 8-10 Ammonium chloride 3.3 oz/gal Boric acid 4.0 ozlgal Table XI I? Qperating Conditions em rature Currie; density 25-50 ASF mfmed Range sulfamate Chloride Bath P 109 1.5 k I l a! Time 10 seconds 5-60 5 6 "9 8 65 Temperature F 70-105 Nickel Chloride 0.6-0.7 oz/gal Currant D ensity 20 l5 25 Boric acid 5.3 oz/gal pH 4.5 Temperature 60F A silver plating or coating thickness of from 0.00003 to 0.00005 inch is normally deposited in about 10 seconds for the indicated current density. As in the preceding deposition such as for the copper and tin, the cathode is the aluminum member held stationary in the electrolyte. The anode is composed of copper, tin, or silver depending upon the metal being deposited.

The eleventh step is the final rinse in hot water after which the member is dried in hot air.

It is noted that although the several rinse steps are preferably used to prevent early contamination of the plating solutions, the rinsing steps may be omitted for the purpose of this invention.

The resulting protective coating provided in the method set forth above provides a corrosion and abrasion resistant plating for stabs or bus ducts when bus bars are subjected to 30,000 consecutive abrasion tests, ie. involving sliding wear.

Wearing tests on aluminum bus bars with and without nickel coating were made for composition. The results of the wear tests are shown in Table Xll as follows:

Table XII No. of

Stab Insertions" Before Coating Wore Through to Coating Process Bare Aluminum" 1. (Alstan 70 process): immersion tin bronze strike +0.0004 inch 4. Same as above but 0.0002 inch nickel 0.00005 inch silver 400 l35 ampere stab contacts plated with 0.00005 inch silvcr A 5% solution of caustic soda was used to determine the presence of bare aluminum Vigorous gassing occurred when this solution was applied to the worn through plate on bare aluminum.

The multiple layered coating of this invention satisties the foregoing problem of fretting with attendent problems. The bronze or copper plating provides a suitable substrate for the nickel strike. The nickel plating has a tri-fold advantage including the establishment of a base for the silver or tin which is subsequently applied, corrosion prevention, and providing a harder base for sliding friction on the stab surface as indicated in Table XII. Silver serves as a lubricant and resists fretting better than tin and does it with a thinner coating.

What is claimed is:

l. A composite article comprising an aluminum alloy member having an abrasion resistant coating on the surface thereof, said coating consisting essentially of an inner layer of a metal selected from the group consisting of tin and zinc, a first intermediate layer of a metal selected from a group consisting of copper and bronze, a second intermediate layer of nickel, and an outer layer of a metal selected from a group consisting of tin and silver.

2. The layer of claim 1 in which the inner coating is tin, the intermediate layer is bronze and the outer layer is tin.

3. The layer of claim 1 in which the inner layer is zinc, the intermediate layer is copper and the outer coating is silver. 

1. A COMPOSITE ARTICLE COMPRISING AN ALUMINUM ALLOY MEMBER HAVING AN ABRASION RESISTANT COATING ON THE SURFACE THEREOF, SAID COATING CONSISTING ESSENTIALLY OF AN INNER LAYER OF A METAL SELECTED FROM THE GROUP CONSISTING OF TIN AND ZINC A FIRST INTERMEDIATE LAYER OF A METAL SELECTED FROM A GROUP CONSISTING OF COPPER AND BRONZE, A SECOND INTERMEDIATE LAYER OF NICKEL, AND AN OUTER LAYER OF A METAL SELECTED FROM A GROUP CONSISTING OF TIN AND SILVER.
 2. The layer of claim 1 in which the inner coating is tin, the intermediate layer is bronze and the outer layer is tin.
 3. The layer of claim 1 in which the inner layer is zinc, the intermediate layer is copper and the outer coating is silver. 